Hybrid switched capacitor circuit with automatic charge balancing

ABSTRACT

An electronic module comprising an input capacitor (C3) connected between a first and second node (N1, N2); a first and second switch (swH, swL) connected in series between said first and second node (N1, N2) and said second node (N2), in parallel with said input capacitor (C3), and defining an intermediate node (N3) at their interconnection; a voltage regulator (VR) configured for receiving power from said input capacitor (C3) and for providing output power at a configurable voltage (out) between an output node (N6) and the second node (N2). A multi-output power supply system comprising three such modules. A LED-driver comprising such module. A multi-colour LED driver comprising three modules. A solid state lighting device comprising three modules and three LEDs. A display device.

FIELD OF THE INVENTION

The present invention generally relates to the field of power convertorcircuits and LED-driver circuits, and more particularly to hybridswitched capacitor circuits (H-SCC) for driving at least three LightEmitting Diodes (LEDs), and to an integrated chip (module) for buildinga multi-LED system, and to a multi-LED system, and to a method ofbuilding such a multi-LED system, and to methods of powering such amulti-LED system.

BACKGROUND OF THE INVENTION

DC-DC converters for generating an output voltage higher or lower thanan input voltage, are known in the art. Examples are: a linearconverter, a buck converter, a switched capacitor circuit (SCC), aswitched inductor circuit (SIC), as well as hybrid circuits where two ormore of the above mentioned converters are used. Each of these circuitshas advantages and disadvantages in terms of power efficiency, maximumpower dissipation, integration level (e.g. single chip, single package,PCB with multiple discrete components), maximum switching frequency,reliability, modularity, etc. DC-DC converters exist in many flavours.It will be appreciated that DC-DC converters for high-powerapplications, e.g. for converting kilowatts or Megawatts, e.g. as can begenerated by wind turbines or solar panel, are completely different fromDC-DC convertors for powering low-power devices from a battery. Thepresent invention is primarily directed to the latter category, whereconsumption of each load to be powered is less than 25 Watt.

The present invention is also related to light emitting diodes,abbreviated as LEDs. A basic knowledge of LEDs is sufficient forunderstanding the principles of the present invention. A goodintroduction to LEDs and LED drivers can be found in chapter 2 of themaster thesis “Hybrid switched converters for CMOS integrated LEDdrivers”, by Castellanos Rodriguez, J. C. (2018), TechnischeUniversiteit Eindhoven, incorporated herein by reference in itsentirety, and referred to as [1], or as “the thesis”.

This thesis also describes hybrid switched capacitor circuits fordriving multi-LED systems. While these circuits are very promising, theystill pose some problems, for example in terms of balancing the inputvoltages of the different modules, and in terms of compactness, andlevel of integration and thus cost. However, the solutions proposed bythe present invention are not limited to light emitting devices, but canalso be used for other circuits where multiple voltages need to begenerated.

There is always room for improvements or alternatives.

SUMMARY OF THE INVENTION

It is an object of embodiments of the present invention to provide anelectronic circuit (also referred to herein as “module”), as abuilding-block for a multi-power supply system (e.g. a multi-voltageDC-DC converter) comprising at least three such modules.

It is also an object of embodiments of the present invention to providea multi-power supply system.

It is an object of embodiments of the present invention to provide asystem comprising at least one module, or at least two modules, or atleast three modules, wherein each module is capable of providing aconfigurable output voltage or output current.

It is an object of embodiments of the present invention to provide sucha system, wherein the output voltages (or output currents) of some orall of the modules can be configured individually and independently ofthe other output voltages (or output currents).

It is an object of embodiments of the present invention to provide amulti-LED driver circuit capable of driving at least two Light EmittingDiodes (LEDs), and/or which may be embedded in a single integratedcircuit (e.g. single semiconductor substrate), or which may be embeddedin a single package (e.g. in the form of an encapsulated semiconductordevice).

It is an object of embodiments of the present invention to provide amulti-LED driver circuit capable of driving at least two Light EmittingDiodes (LEDs) independently.

It is an object of embodiments of the present invention to provide amulti-LED driver circuit capable of driving at least two Light EmittingDiodes (LEDs) configured for transmitting light of different colours ordifferent wavelengths.

It is an object of embodiments of the present invention to provide amulti-LED driver circuit capable of dimming the light generated by theLEDs.

It is an object of embodiments of the present invention to provide anelectronic device comprising a multi-LED driver circuit and at least twoLight Emitting Diodes (LEDs), for example in the form of a single chipor a single package comprising one or more semiconductor dies.

It is an object of embodiments of the present invention to provide asingle-chip or single package comprising: a tri-color LED driver circuitand three LEDs of different colour or different wavelength.

It is an object of embodiments of the present invention to provide amodule (e.g. in the form of a single chip) for building such a multi-LEDdriver circuit in a modular manner, for example by using three suchmodules and a small number of discrete components, e.g. at most twocapacitors and three inductors and three LEDs.

It is an object of embodiments of the present invention to provide sucha multi-LED driver circuit and/or such a module capable of driving saidmultiple LEDs with high efficiency, e.g. at least 85% or at least 90%,or at least 92%, or at least 94%.

It is an object of embodiments of the present invention to provide sucha module in the form of a single chip (i.e. semiconductor die) which ismore compact.

It is an object of embodiments of the present invention to provide sucha module implemented in standard CMOS technology (for example 0.18micron).

It is an object of embodiments of the present invention to provide sucha module configured for being powered by a voltage in the range fromabout 5.0V to about 7.0V.

It is an object of embodiments of the present invention to provide sucha system configured for being powered by a DC supply voltage in therange from about 7.5 V to about 10V, or from about 7.5V to about 24V, orfrom about 7.5V to about 50V, or from 7.5V to about 200V.

According to a first aspect, the present invention provides anelectronic circuit comprising: an input capacitor connected between afirst node and a second node; a first switch and a second switchconnected in series between said first node and said second node, inparallel with said input capacitor, and defining an intermediate node attheir interconnection; a voltage regulator configured for receivingpower from said input capacitor and for providing output power at aconfigurable voltage between an output node and the second node; whereinthe first and second switch is configured to be toggled at a frequencyof at least 2 MHz, or at least 3 MHz, or at least 4 MHz, or at least 5MHz, or at least 6 MHz, or at least 7 MHz, or at least 8 MHz, or atleast 9 MHz, or at least 10 MHz.

It is an advantage of such a module that it can be used to build LEDdriver circuits in a variety of series and/or parallel configurations,with every individual LED being controllable to any power level.

It is a major advantage of this module that it allows easyimplementation of “auto-balancing”, and thereby enables cost-effectivemassive pixelation.

In an embodiment, the electronic circuit further comprising an outputcapacitor connected between the output node and the second node forstabilizing the output voltage.

In an embodiment, the electronic circuit further comprises a controlunit configured for receiving a switch control signal via a fourth node,and for generating a first switch signal to control the first switch,and for generating a second switch signal to control the second switch.

In an embodiment, the control unit comprises a finite state machineand/or a level shifter.

In an embodiment, the electronic circuit further comprises a fifth nodefor passing the switch control signal or a signal derived therefrom toanother electronic circuit.

In an embodiment, the control unit is further configured for receivingan output control signal via a fourth node and for providing this outputcontrol signal, or a signal derived therefrom to the voltage regulatorto control the configurable voltage.

The control unit may comprise a serial interface for receiving saidoutput control signal, and may comprise a PWM-generator for providing apulse-width-modulated signal to the voltage regulator or adigital-to-analog converter for providing an analog voltage signal tothe voltage regulator for controlling the output voltage to begenerated.

In an embodiment, the electronic circuit further comprises a lightemitting diode connected at the output of the voltage regulator.

It is an advantage that each LED is provided with its own driver, fullyor almost fully integrated (apart from a small number of discretecomponents) in a standard low-voltage and low-cost IC process, e.g.standard CMOS process.

In an embodiment, the voltage regulator is a linear voltage regulator.

In an embodiment, the voltage regulator is a switched inductor converter(SIC).

In an embodiment, the voltage regulator is a resonant or a hybridswitched capacitor converter.

In an embodiment, the voltage regulator is a resonant switched capacitorconverter or a hybrid switched capacitor converter comprising at leastone capacitor and at least one inductor connected in series with saidcapacitor, wherein the capacitor has a value in the range from 400 pF to1.4 nF; and wherein the inductor has a value in the range from 40 nH to160 nH.

In an embodiment, the voltage regulator is a resonant switched capacitorconverter or a hybrid switched capacitor converter comprising at leasttwo switches, configured to switch at a frequency in the range from 20MHz to 60 MHz.

According to a second aspect, the present invention also provides asystem comprising: a first, second and third module according to thefirst aspect, wherein the first node of the second module is connectedto the second node of the first module, and wherein the intermediatenode of the second module is connected to the second node of the thirdmodule; and wherein the intermediate node of the first module isconnected to the first node of the third module; and wherein the firstand second switch of at least the first and second module is beingtoggled at a balancing frequency of at least 2 MHz, in such a way that:during a first moment in time, the first switch of the first and secondmodule are configured to be closed while the second switch of the firstand second module are configured to be open, and during a second momentin time, the first switch of the first and second module are configuredto be open while the second switch of the first and second module areconfigured to be closed, thereby causing charge distribution between theinput capacitor of the first, and second and third module.

This embodiment can be considered to be a “power distribution system”capable of providing power at three different voltages. When used fordriving LEDs, this circuit can be considered to be a “multi-LED driver”.

It is an advantage of this toggling of the switches, that a supplyvoltage, applied to the first node of the first module is substantiallyequally divided over the three modules. This technique is referred toherein as “auto-balancing”, and is one of the underlying principles ofthe present invention.

It is an advantage that the toggling frequency is not critical, providedit is sufficiently large (e.g. at least 2 MHz).

It is a further advantage that the duty cycle of the signals provided tothe switches does not have to be 50% either, but can for example be avalue in the range from 10% to 40%.

In an embodiment, each module comprises a light emitting diode.

This embodiment is a “multi-LED system”, for example, if the colours ofthe three LEDs are different, a tri-color LED system.

In preferred embodiments, this system is incorporated in a singlepackage. The package may comprise three chips corresponding to the threemodules, and only a small number of discrete components, for example twodiscrete capacitors (e.g. C1 and C2 in FIG. 1 a ) and optionally threeinductors (e.g. Lx of FIG. 6 or Lx of FIG. 8 ).

It is an advantage of this system that the LED drivers areinterconnected (e.g. stacked) in such a way that the available systemsupply voltage automatically distributes equally over all the modules orstacked chips or LEDs, even when LEDs of different type or manufacturerare used, and even in case the power per LED differs largely.

In an embodiment, the system further comprises at least three discretelight emitting diodes, each connected to an output (N6) of one module.

This embodiment is a “multi-LED system”, for example, if the colour ofthe three LEDs are different, a tri-color LED system.

In an embodiment, the system further comprises a system controllerconfigured for providing a switch control signal of at least 2 MHz to atleast the first and the second module.

In an embodiment, the system controller is further configured forproviding a first output control signal to the first module, and asecond output control signal to the second module, and a third outputcontrol signal to the third module.

The output control signal may be a single digital bitstream, containingat least three digital values defining the output of the first, secondand third module.

According to a third aspect, the present invention also provides amethod of building a solid state light emitting device, comprising thesteps of: a) providing at least a first, a second and a third moduleaccording to the first aspect; b) connecting the first node of thesecond module to the second node of the first module; c) connecting theintermediate node of the second module to the second node of the thirdmodule; d) connecting the intermediate node of the first module to thefirst node of the third module.

According to a fourth aspect, the present invention also provides amethod of powering three light emitting diodes, using a system accordingto the second aspect, the method comprising the steps of a) providing asupply voltage over the first node of the first module, and the secondnode of the second module; b) toggling the first and second switch of atleast the first and second module at a balancing frequency of at least 2MHz, in such a way that: during a first moment in time, the first switchof the first and second module are configured to be closed while thesecond switch of the first and second module are configured to be open,and during a second moment in time, the first switch of the first andsecond module are configured to be open while the second switch of thefirst and second module are configured to be closed; thereby causingcharge redistribution between the input capacitor of the first, secondand third module.

The supply voltage (for a 3 module system) may be a voltage in the rangefrom 6.0V to 10.0V.

According to a fifth aspect, the present invention also provides adisplay device comprising a plurality of pixels organized in rows andcolumns, the display device comprising a plurality of systems accordingto the second aspect, each system forming one pixel of said displaydevice.

These and other aspects of the inventions will be apparent from andelucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) shows an illustrative block diagram of a multi-output powersupply system according to an embodiment of the present invention,having at least one “floating” output. FIG. 1(b) illustrates chargedistribution during a first phase. FIG. 1(c) illustrates chargedistribution during a second phase.

FIG. 2 shows simulation results of “voltage balancing” of the circuit ofFIG. 1(a), for a first set of equal loads (5Ω//1 μF), when applying 7.5Vsupply voltage, using a toggle frequency of 20 MHz and a duty cycle ofabout 25%. FIG. 4(a) shows the voltages over the three input capacitors.FIG. 4(b) shows control signals applied to the switches.

FIG. 3 shows simulation results of “voltage balancing” of the circuit ofFIG. 1(a), for three different loads (load1=5Ω//1 μF, load2=10Ω//1 μF,and load3=15Ω//1 μF), when applying 7.5V supply voltage, using a togglefrequency of 2 MHz (10 times lower than the 20 MHz switching frequencyof the modules) and a duty cycle of about 25%.

FIG. 3(a) shows the voltages over the three input capacitors.

FIG. 3(b) shows control signals applied to the voltage balancingswitches (swnH, swnL, nϵ{1 . . . 3}).

FIG. 4(a) shows an electronic circuit or module according to anembodiment of the present invention, three of which are used in themulti-output power supply system of FIG. 1(a).

FIG. 4(b) shows a variant of the module of FIG. 4(a), further comprisinga light emitting diode (LED), according to an embodiment of the presentinvention.

FIG. 4(c) shows a variant of the module shown in FIG. 4(a), furthercomprising a control circuit, according to an embodiment of the presentinvention.

FIG. 4(d) shows a variant of the module of FIG. 4(c), further comprisinga light emitting diode (LED), according to an embodiment of the presentinvention.

FIG. 5(a) shows a block-diagram of a multi-LED driver circuit accordingto an embodiment of the present invention, comprising three modules asshown in FIG. 4(b), each connected to a LED.

FIG. 5(b) shows a block-diagram of a multi-LED driver circuit accordingto an embodiment of the present invention, comprising three modules asshown in FIG. 4(d).

FIG. 6 shows a simplified circuit diagram of a proposed switchedinductive voltage regulator circuit as can be used in the modules andthe multi-output power supply systems of FIG. 1 to FIG. 5 .

FIG. 7 shows simulation results for the circuit of FIG. 6 .

FIG. 8 shows a variant of the voltage regulator circuit illustrated inFIG. 6 , where the tank is tuned to a higher harmonic resonantfrequency.

FIG. 9 shows simulation results for the circuit of FIG. 8 .

The drawings are only schematic and are non-limiting. In the drawings,the size of some of the elements may be exaggerated and not drawn onscale for illustrative purposes.

DETAILED DESCRIPTION

The present invention relates to DC-DC converter circuits, and more inparticular to hybrid switched capacitor circuits capable of powering atleast two or at least three electrical loads. It is a challenge toprovide reliable circuits which are both compact and highly energyefficient.

The present invention also relates to LED driver circuits and multi-LEDdriver circuits, as an example of such circuits where the load is alight emitting diode (LED). The LEDs should be individually dimmable toallow generation of various colours.

As described in the background section, the PhD thesis [1] describesvarious circuits and sub-circuits which can be used in such modules orsystems. In chapter 6 of this thesis multi-LED driver circuits aredescribed, composed of multiple hybrid switched capacitor modulesconnected in parallel or in series. As stated in paragraph 6.3.2, thebalancing of the different modules remains a technical challenge, whichis not solved.

The present invention provides a solution to that problem, by providingan auto-balancing technique by using at least three modules having aninput capacitor and a first pair of switches, as will be illustratedmainly in FIG. 1 to FIG. 3 . The modules furthermore comprise a voltageregulator which is powered by the input capacitor. In principle any typeof voltage regulator can be used.

FIG. 4 and FIG. 5 illustrate a modular approach, proposed by the presentinvention, which is surprisingly convenient for embodiments of thepresent invention. Moreover, such modular approach allows to reduce thedesign and test effort considerably.

In some embodiments of the present invention, the voltage regulator is aswitched inductor circuit (SIC) containing a second pair of switches anda resonant circuit, as will be illustrated for example in FIG. 6 andFIG. 7 . In this case, preferably zero-current-switching (ZCS) mode ofoperation is used. In Zero-Current Switching (ZCS), the switches arecommutated at zero current. This soft-switching technique decreases theswitching losses in the switches, and thus increases power efficiency.

The present invention also provides an embodiment where the voltageregulator inside the module is a switched inductor circuit (SIC)containing a resonant circuit configured to resonate at an integermultiple (e.g. M=3) of the switching frequency of the second pair ofswitches, as will be illustrated for example in FIG. 8 and FIG. 9 .

Referring now to the Figures.

FIG. 1(a) shows a simplified block diagram of a stack or seriesconnection of three modules M1, M2, M3 forming a multi-output powersupply system 100.

In the example of FIG. 1(a), the stack contains three modules M1, M2,M3, suggested by the dotted lines, but the present invention is notlimited to systems containing only three modules, and will also work formore than three modules. Each module Mi has a voltage regulator VRi. Inthe example of FIG. 1 , each modules also contains a load as part of themodule, but that is not absolutely required, and the load may also beexternal to the module. In the example of FIG. 1 a , each load is acapacitor in parallel with a resistor, but the present invention is notlimited thereto. In some embodiments of the present invention, theresistors are replaced by a light emitting diode (LED), as will bedescribed further. The modules may be implemented as discrete 7V ICs,meaning, designed to be powered by a voltage of at most 7V.

The main purpose of FIG. 1(a) to FIG. 1(c) is to illustrate theprinciple of “load balancing” proposed by the present invention, whichis achieved by adding three capacitors C1, C2, C3 and at least two pairsof switches in front of the voltage regulators, namely a first capacitorC1 and a first pair of switches sw1H, sw1L in front of the first voltageregulator VR1 of the first module M1; and a second capacitor C2 and asecond pair of sw2H, sw2L in front of the second voltage regulator VR2of the second module M2; and a third capacitor C3 in front of the thirdvoltage regulator VR3 of the third module M3. It is noted that the thirdmodule M3 may also have a pair of switches sw3H, sw3L, but these are notused in practice, and may be omitted (e.g. in a dedicatedimplementation).

As can be seen in FIG. 1(a), the capacitors C1 and C2 of the first andsecond module M1, M2 are connected in series between the supply voltageVCC and ground GND, whereas the terminals of the capacitor C3 of thethird module M3 are connected to a node “y” defined by theinterconnection of the switches sw1H, sw1L of module M1, and a node “z”defined by the interconnection of the switches sw2H, sw2L of module M2.

According to an aspect of the present invention, balancing of thevoltages over the capacitors C1, C2, C3 is obtained by toggling theswitches of the first and second module M1, M2 at a frequency of atleast 10 MHz, e.g. at a frequency in the range from 10 MHz to 20 MHz,e.g. at a frequency of about 11 MHz, or about 12 MHz, or about 13 MHz,or about 14 MHz, or about 15 MHz, or about 16 MHz, or about 17 MHz, orabout 18 MHz, or about 19 MHz. The switches need to be toggled in such away that:

i) during a first time period (or phase), the high-side switches sw1H,sw2H of the first and second module M1, 2 are closed while the low-sideswitches sw1L, sw2L are open, as illustrated in FIG. 1(b). As can beseen, in this configuration the capacitor C3 of the third module M3 isconnected in parallel to the capacitor C1.

ii) during a second time period (or phase), the high-side switches sw1H,sw2H of the first and second module M1, 2 are open while the low-sideswitches sw1L, sw2L are closed, as illustrated in FIG. 1(c). As can beseen, in this configuration the capacitor C3 of the third module M3 isconnected in parallel to the capacitor C2.

This toggling causes charge redistribution between the three capacitorsC1, C2, C3 in such a way that the voltage over each of these capacitorsis substantially equal, provided that the capacitor value of C1, C2 andC3 are equal. Preferably the toggling frequency of the input capacitorsis sufficiently high (e.g. at least 2 MHz, or at least 5 MHz, or atleast 10 MHz) to limit or reduce losses caused by relatively largevoltage imbalances.

Thus, as an example, if the supply voltage Vcc is equal to 10V, thebalancing scheme ensures that the voltage over C1, C2 and C3 issubstantially equal to 10V/2=5V. As another example, if the supplyvoltage Vcc is equal to 7.5V, the balancing scheme ensures that thevoltage over C1, C2 and C3 is substantially equal to 7.5V/2=3.75V.

This can be understood as follows: the module M1 generating “out1”presents an effective impedance “r1” across the nodes “in” and “x”, andis connected in series with the module M2 generating “out2”, whichitself presents an effective impedance “r2 across node “z” and ground.The voltage across the capacitors C1 and C2 therefore stabilizes atVcc·r2/(r1+r2). When controlling the power on “out1” and “out3” toarbitrary values, the effective impedances “r1” and “r2” will not beequal, and the voltage on node “x” will differ from the desired 5V (50%of the 10V supply voltage) that is needed for correct independentoperation. By adding the third module M3, fed from the capacitor C3, andby connecting C3 rapidly back and forth between the nodes (in, x) and(x, ground), the voltage on C3 is forced to be equal to the voltage onC1, then to the voltage on C2, then to the voltage on C1, etc. In theend, the three voltages have to be equal. Since the sum of the voltagesacross C1 and C2 is equal to the supply voltage Vcc, e.g. 10V in theexample, in steady-state the voltage over the three modules M1, M2, M3stabilizes to approximately 5V. By choosing a sufficiently high togglingfrequency (e.g. at least 10 MHz) and by choosing capacitor impedancessufficiently low relative to r1 and r2, it can be ensured that thecapacitors do not discharge significantly between two toggling phases.It is noted that, the lower the voltage imbalance, the lower the energylost in the capacitive charge/discharge process.

For completeness, it can be seen that the switches sw3H and sw3L of thethird module M3 could be omitted, but in a modular approach, theseswitches would be present in every module. In an embodiment, theswitches sw3H, sw3L could both remain open at all times (which mayreduce switching losses), but since the intermediate node of the thirdmodule M3 is not connected to any of the capacitors C1, C2, C3, theswitches sw3H, sw3L may also be toggled along with the switches of theother modules.

FIG. 2 shows simulation results of “voltage balancing” of the circuit ofFIG. 1(a), for a first set of equal loads (5Ω//1 μF), when applying 7.5Vsupply voltage, using a toggle frequency of the switches sw1H, sw1L,sw2H, sw1L of 2 MHz and a duty cycle of about 25%. The input capacitorsC1, C2 and C3 are 1 μF. FIG. 2(a) shows the voltages over the threeinput capacitors. FIG. 2(b) shows control signals applied to the toggleswitches. In this simulation the voltage regulator blocks are switchingat 20 MHz, as evidenced by the ripple on the output voltages.

As can be seen in FIG. 2 , the voltage balancing works surprisinglywell. The three output voltages (here chosen to be substantially equalto 2.5V) are balanced within approximately 800 ns to their nominalvalue. The maximum steady state error between the outputs is about 45mV.

FIG. 3 shows simulation results of “voltage balancing” of the circuit ofFIG. 1(a), for three different loads (load1=5Ω//1 μF, load2=10Ω//1 μF,and load3=15Ω//1 μF), when applying 7.5V supply voltage, using a togglefrequency of the switches sw1H, sw1L, sw2H, sw1L of 2 MHz and a dutycycle of about 25%. The input capacitors C1, C2 and C3 are 1 μF. FIG.3(a) shows the voltages over the three input capacitors. FIG. 3(b) showscontrol signals applied to the switches.

As can be seen from FIG. 3 , the voltage balancing still workssurprisingly well, even for a 3 to 1 load unbalance. The three outputvoltages (again chosen to be substantially equal to 2.5V) are againbalanced within approximately 800 ns to their nominal values. In thisexample, the final uncorrected voltage error is about 90 mV. This errorcan be reduced by choosing bigger input capacitors C1, C2, C3 and/or byincreasing the toggling frequency.

As connecting capacitors in parallel (known as “paralleling capacitors”)causes a nearly instantaneous charge equalization, the duty cycle of thetoggling signal need not be 50%. In practice the time constant of theON-resistance of the switch and the total capacitance determines thelength of the minimum charging interval. Somewhat surprisingly a dutycycle of about 10% appears to substantially minimize the output voltagedifferences. It was found that the output voltages difference for a dutycycle of 25% are negligible.

From the above, it can be understood that the “auto-balancing principle”allows interconnecting modules (e.g. chips) in such a way that theavailable system supply voltage automatically distributes evenly overall the modules (e.g. stacked chips) for powering various loads, e.g.LEDs, even when LEDs of different type or manufacture are used and/orwhen the power per LED differs greatly. But the present invention is notlimited to LEDs, and other loads, such as e.g. sensors or detectors ortransducers can also be used.

Preferably the modules are manufactured in a low voltage process, e.g. a7V process. The proposal of FIG. 1 also makes it possible that themodules (e.g. stacked chips) directly communicate over a wired bus, e.g.a serial bus, manufactured in the same 7V process.

Referring back to FIG. 1(a), it can be understood that the balancingscheme will work for various kinds of voltage regulators VR. Inembodiments of the present invention, the modules M1, M2, M3 comprise alinear voltage regulator, or a switched capacitor circuit (SCC), or aswitched inductive circuit (SIC), or any other suitable voltageregulator. Depending on the voltage regulator type, a different controlsignal will be applied to the control port “mod” of the voltageregulator. In the example of FIG. 1(a) this control signal isrepresented as a DC voltage, as can for example be generated by a(local) digital-to-analog-convertor (DAC, not shown), but the presentinvention is not limited thereto, and other control signals forcontrolling the voltage regulator for adjusting the output voltage canalso be used, for example a pulse-width modulated (PWM) signal (notshown). Such an analog voltage or a PWM signal may for example begenerated by a control circuit as shown in FIG. 4(c) or FIG. 4(d). Thecontrol signal may be an analog circuit, or a digital circuit, or ahybrid circuit (with an analog portion and a digital portion), and maycontain for example a finite state machine (FSM), etc. The actual“setting” or “configuration” or “command” of the output powers (oroutput voltages) of each of the modules may come e.g. from a systemcontroller external to the multi-power supply system, for example asillustrated in FIG. 5(a) or FIG. 5(b).

Referring back to FIG. 1(a) once more, it can be understood that theswitches sw1H, sw1L of the first module M1, and the switches sw2H, sw2Lof the second module M2 need to be toggled synchronously. This can beachieved for example by providing a switch control signal (e.g. by thesystem controller mentioned above) to the second module M2, which signalis locally converted into two complementary signals for controlling thelocal switches, and which signal is daisy chained towards the firstmodule M1, optionally via the third module M3.

In the embodiment shown in FIG. 1 , a single control line isdaisy-chained from module to module (e.g. from chip to chip). This linemay (digitally) communicate a different value to each module.Alternatively, this line may provide a synchronous charge balancingsignal toggling at a frequency below 25 MHz, e.g. in the range from 2MHz to 20 MHz, or in the range from 10 MHz to 20 MHz.

Next, the modularity of the system of FIG. 1(a) will be described inmore detail.

As already suggested above, the multi-power supply system of FIG. 1(a),or variants thereof, e.g. as shown in FIG. 5(a) and FIG. 5(b), can bebuild using a plurality of at least three identical modules M1, M2, M3.While the block-diagram of FIG. 1(a) suggests that the capacitors C1,C2, C3 are located outside of the modules M1, M2, M3, the inventorsrealized that in fact, the capacitors can actually be incorporated inthe module itself. This offers an important advantage, because in thisway the number of discrete components can be reduced.

FIG. 4(a) shows a block-diagram of an exemplary “building block” or“module” proposed by the present invention. This module is preferablyimplemented as a single chip, with as few external components aspossible. The module 410 comprises:

-   -   a first node N1;    -   a second node N2;    -   an input capacitor C3 connected between node N1 and N2;    -   a high-side switch swH and a low-side switch swL, connected in        series between node N1 and N2, and defining an intermediate node        N3 between the two switches;    -   a voltage regulator VR having an input port connected to node        N1, a sink (or ground) port connected to node N2, a power output        port “out” (node N6), and a control input “mod”;    -   optionally an output capacitor C4 connected between the output        of the voltage regulator VR and node N2. It is noted that this        output capacitor may be omitted, depending on the load that will        be connected to the voltage regulator VR;    -   a control voltage generator (schematically indicated by the        circle with a plus and minus sign) located inside the module,        for controlling the voltage regulator VR of this particular        module. The control voltage generator can for example comprise a        digital-to-analog-convertor (not explicitly shown) connected to        control logic (not shown). The signal to be generated by this        control voltage generator can for example be set by an external        processor (not shown);

As already mentioned above, in principle, any type of voltage regulatorcan be used, for example a linear regulator, or a switched inductivecircuit (SIC), or a switched capacitor circuit (SCC). A linear regulatoroffers the advantage that it can be fully integrated on a semiconductordie (no external components), and that its output voltage can be easilycontrolled by means of an analog voltage, but has the disadvantage thatthe power efficiency is not optimal. The same advantages anddisadvantages also apply for a switched capacitor circuit (SCC). In apreferred embodiment, however, the voltage regulator is a switchedinductive circuit (SIC) with a relatively small inductance. A particularexample will be described in more detail in FIG. 6 .

The switches swH and swL are controlled by one or two control lines, notshown in detail, but schematically illustrated by a dotted line runningbetween node N4 and N5. The individual control signals for each switchare preferably generated inside the module in manners known per se inthe art, on the basis of a switch-control-signal which is daisy-chainedbetween the different modules.

FIG. 4(b) shows a block-diagram of a module 420 similar to the module410 of FIG. 4(a), furthermore including a light emitting diode LED,preferably integrated in the module, or connected (e.g. soldered) to themodule.

FIG. 4(c) shows a block-diagram of a module 430, which can be considereda variant of the module 410 of FIG. 4(a), furthermore comprising acontrol circuit (e.g. an analog or digital or mixed analog and digitalcircuit) configured for receiving the above described “switch controlsignal” and/or the above described “output control signal”, andconfigured for providing a first signal to the high-side switch swH, asecond signal to the low-side switch swL, a third signal to the voltageregulator, and a fourth signal to the node N4.

FIG. 4(d) shows a block-diagram of a module 440 similar to the module430 of FIG. 4(c), furthermore including a light emitting diode LED,preferably integrated in the module in the form of a semiconductor die,or connected (e.g. soldered) to the module.

FIG. 5(a) shows a block-diagram of a multi-LED driver circuit 510,comprising three modules as shown in FIG. 4(b), each module arranged forbeing connected to a corresponding LED. If the LEDs are present, thecircuit of FIG. 5(a) inside the dashed rectangle shows a solid statelighting device, or a multi-LED system 510, e.g. a tri-color LED system.As can be seen, the input capacitor of the first module M1 is labelled“C1”, and the input capacitor of the second module M2 is labelled “C2”.

In an embodiment, the multi-LED system 510 comprises three modules M1,M2, M3 fully integrated on a single semiconductor die (thus one singledie comprising the three modules), the semiconductor die being connectedto three discrete LEDs. The semiconductor die and the three LEDs beinginterconnected and being packaged into a single package (i.e. packagedcomponent). As shown, this multi-LED system can be easily connected to avoltage supply VCC and a system controller, external to the multi-LEDsystem. In such a system, the light output (intensity) and color can beset by the external system controller.

In a variant, the three modules are almost completely integrated on asingle semiconductor die, except for three inductors. These inductorsmay be implemented as discrete components, or as conductive tracks (e.g.copper tracks) on a substrate (e.g. a printed circuit board)electrically connected to the single semiconductor die. The substratewith the inductors or with the inductive tracks may be packaged into asingle package (i.e. packaged component).

FIG. 5(b) shows a block-diagram of a multi-LED system according to anembodiment of the present invention, comprising three modules as shownin FIG. 4(d). This system is a variant of the system shown in FIG. 5(a)where the LEDs are integrated in the module, e.g. implemented on therespective semiconductor die as the rest of the module.

In an embodiment, the multi-LED system 520 comprises three modules M1,M2, M3 fully integrated on a single semiconductor die (thus one singledie comprising the three modules). As shown, this multi-LED system canbe easily connected to a voltage supply VCC and a system controller,external to the multi-LED system.

In a variant, the three modules are almost completely integrated on asingle semiconductor die, except for three inductors. These inductorsmay be implemented as discrete components, or as conductive tracks (e.g.copper tracks) on a substrate (e.g. a printed circuit board)electrically connected to the single semiconductor die.

Thus FIG. 5(b) illustrates a fully integrated on-chip or in-package LEDdriver using the H-SCC approach with integrated inductor and capacitors(“integrated” meaning: embedded in the semiconductor substrate, orencapsulated in the package).

FIG. 6 shows a simplified circuit diagram of a proposed switchedinductive voltage regulator circuit as can be used in the systemsdescribed above. For clarity, the auto-balancing switches are not shown.It is noted that in a practical realisation, diodes can be implementedusing transistors, a detail not shown here.

The capacitor C8 and the inductor L1 connected in series with C8 form atank. The quality factor Qm of the tank can be chosen (during design) tobe a value in the range from about 0.33 to about 1.0. Depending on thequality factor Qm, the values of the other components can be determined,for example as indicated in FIG. 6 .

The circuit of FIG. 6 is a resonant switched capacitor circuit (ReSC)resembling the one proposed in [1], on page 60, FIG. 4.1 , but uses zerocurrent switching (ZCS) of the switches sw4 and sw5 at approximately theresonance frequency of the tank. The switch duty-cycle is kept constant.It is noted that the PWM circuit in this simulation was used to find anoptimal value w.r.t. circuit tolerances, but it can also be used as an“analog” controller.

The driver of FIG. 6 internally uses auto-synchronizedzero-current-switching at more than 25 MHz. If this driver is used inmodule #i of a system, the output #i of that system can be controlled byturning the converter either fully ON or OFF for an integer number Ni ofclock cycles.

As the switching frequency of the switches sw4, sw5 in this example isset to a frequency of about 25 MHz, very high resolution dimming ispossible by turning individual LEDs fully ON or OFF for integer numbersof switching cycles. This has the added advantages that the efficiencyof the driver is optimal over the full power range, and that an easyrealization is possible even for much higher switching frequencies (e.g.up to 100 MHz). In addition to the on/off scheme, the LEDs can befine-tuned in an analog fashion over a limited range by altering theswitching frequency. This will not decrease the power efficiency whenthe switches are driven in discontinuous ZCS mode, i.e. when theswitching frequency is kept below the resonance frequency of the tank.

For completeness, it is noted that the circuit shown in FIG. 6 differsmainly from the topology of the circuit in the thesis (FIG. 4.1 ) inthat the inductor is in series with the load, not with the capacitor.Also, in the thesis a complicated form of PWM is used to regulate theLED current, while in the circuit of FIG. 6 of the present invention,the voltage converter is always in ZCS, slightly in discontinuous mode(i.e. f_(sw) less than the resonant tank frequency) to decrease theinductor size further, and to be less dependent on component tolerances.Furthermore, in FIG. 6 , the output is preferably controlled by turningthe converter fully ON for a number of cycles, then OFF for anothernumber of cycles (“subharmonic PWM”). It is an advantage of working indiscontinuous mode, and using on-off control mode rather than trying toregulate the current each cycle (which is more complicated).

FIG. 7 shows a simulation of a tank current (i.e. the current flowingthrough the inductance Lx of FIG. 6 ), and the voltage of the proposedswitched inductive circuit of FIG. 6 . The top trace of FIG. 7(a) showsan exemplary waveform of the current flowing through the switches. Thebottom trace of FIG. 7(c) indicates that the expected efficiency isabout 98.5%, assuming 100 mΩ switches. This is a big advantage of thiscircuit. The trace of FIG. 7(b) shows the voltage over the capacitor C8and the current flowing through the inductor Lx as a function of time.As can be seen, the peak-to-peak voltage over the capacitor C8 is about9V, and the current has a magnitude of about 1200 mA.

This mode of operation allows to fine-tune or adjust the output voltageby means of the switching frequency instead of the duty-cycle. In casethe load is a LED, only very small voltage and therefore frequencychanges are necessary, because of the exponential behaviour of currentflowing through the LED versus voltage over the LED. For larger outputpower variations, it is proposed to turn OFF the voltage regulatorcircuit for an integer number of switching cycles, e.g. by providing alogical ‘0’ to the gate of both switches sw4 and sw5, to open bothswitches.

Apart from the high energy efficiency of this scheme, another bigadvantage of the voltage regulator of FIG. 6 is that the on-chipresonant capacitor Cs is only 444 pF (for a resonance frequency of 20MHz and Qm=1) instead of 7.75 nF needed in the prior art circuit (at 18MHz), resulting in 17 times less chip area (e.g. requiring only about0.2 mm², assuming a technology offering 2.2 fF/μm²). Instead of theLx=220 nH external inductor needed in the prior art circuit [1], thecircuit proposed herein only needs an inductor of Lx=150 nH. Such aninductor can be implemented as a discrete inductor (external to thesemiconductor die), or internal (e.g. using the parasitic bond wireinductance) or as copper track in the form of a loop (also external tothe semiconductor die), or in any other suitable way.

It is noted that running with Qm=1 means that the voltage on theinternal capacitor Cs can become 24Vpp (peak-to-peak). If Qm is chosenequal to 0.33, the peak-to-peak voltage over the internal capacitor Cscan be reduced to 7 Vpp, at the cost of a three times bigger capacitanceCs=1.35 nF, (requiring about 0.6 mm²) but with the advantage of a threetimes smaller inductance Lx=50 nH (smaller internal or externalcomponent).

But of course, the present invention is not limited to Qm-values equalto 0.33 or equal to 1.0, but the skilled person can choose other valuesof Qm in the range from 0.33 to 1.0.

Such a voltage regulator in the form of a resonant switched capacitorcircuit (ReSC) can for example be implemented as a single-chip in CMOStechnology, requiring only one external component, namely the inductorLx, having a value in the range from about 50 nH to about 150 nH. But asalready mentioned above, depending on technology, an internal bond wireinductance may replace the external inductor, thereby reducing thenumber of external components.

FIG. 8 shows a variant of the voltage regulator circuit illustrated inFIG. 6 , where the tank is tuned to a higher harmonic frequency, forexample to the third harmonic frequency (M=3). With M=3 both thecapacitor Cs and the inductor Lx become three times smaller,respectively Cs=150 pF and Lx=50 nH. However, the voltage over Cs wouldbecome 27 Vpp, and may become even higher when the peak current isincreased to compensate for the lower output power. Importantly, thedecreased tank component values make a very small and cost effective ICpossible. The downside is that the switch current needs to be higher forthe same output power.

Such a voltage regulator in the form of a resonant switched capacitorcircuit (ReSC) operating at the 3rd harmonic, may be implemented in CMOStechnology or in GaN technology, requiring only one external component,namely the inductor Lx, having a value of about 40 to 60 nH, e.g. equalto about 50 nH. Again, depending on technology, an internal bond wireinductance may replace this external inductor.

A small disadvantage of the on-off control mode is a higher currentripple, resulting in larger output capacitors of about 100 nF, but thisis still much smaller than the output capacitor of 10 μF, used in [1].

It is noted that the output of the resonant circuit of FIG. 6 and FIG. 8can be turned completely ON (by toggling the switches) for a firstpredetermined time period, or completely OFF (by opening both switchessw4, sw5) for a second predetermined time period. These time periods canbe counted as an integer number of oscillation cycles, and thus allows afully digital, high-resolution ON-OFF control scheme. This leads to highenergy efficiency (ratio of light versus power consumption). By choosinga suitable first and second predetermined time period, any desired dutycycle and thus light intensity level can be generated. It is noted thatthis duty cycle can be chosen substantially independently from theswitching frequency of the balancing switches sw1H, sw1L, sw2H, sw2Ldescribed in FIG. 1(a). Furthermore, since the ON and OFF cycles can bearbitrarily distributed over an update cycle (“dithering”), visualaberrations can be reduced.

FIG. 9 illustrates the behaviour of the voltage regulator of FIG. 8 .

FIG. 9(a) shows the voltage swing from about +17.5V to about −7.5V,resulting in the above mentioned 27 Vpp. FIG. 9(b) illustrates anexemplary light emission when using the circuit of FIG. 8 . FIG. 9(c)illustrates both the instantaneous input power of the voltage regulatorP_(in), and the instantaneous power efficiency η (eta). FIG. 9(d)illustrates the voltage across and the current through the two regulatorswitches, which is a measure for the switching losses.

While the present invention is described with reference to particularexamples, the present invention is not limited thereto, and othervariations to the disclosed embodiments can be understood and effectedby those skilled in the art in practicing the claimed invention, from astudy of the drawings, the disclosure, and the appended claims.

For example, while most drawings are shown with three modules, eachdriving a respective load, the present invention also works for a systemhaving three modules but only two loads. In this case, the third modulewould not drive a load but is used for balancing the input voltages.

While the three loads in one example were three resistors, and inanother example were three LEDs, the present invention is not limited toloads of the same type, can also be used to drive different kinds ofloads, such as for example LEDs transmitting visible light, UV LEDs,LiFi, radar or other detectors or sensors.

In some embodiments described above, the modules, in particular thevoltage regulators are described to be controlled by means of a wiredconnection, but that is not absolutely required, and the voltageregulators may also be controlled wirelessly. In this case, the controlcircuit would further comprise a transceiver circuit (not shown), forexample based on Bluetooth or ZigBee, or any other wirelesscommunication standard.

In the embodiments described above, three modules are interconnectedsuch that the supply voltage is divided by two. In such configuration,the input capacitors C1, C2 of the first and the second module areconnected in series, and the input capacitor C3 of the third module isalternatingly connected to the input capacitor of the first module andthe second module. But the present invention is not limited to systemshaving three modules, and systems according to the present invention canalso have more than 3 modules, for example 5 modules. In this case, theinput capacitors of three modules would be connected in series (firststage), while the input capacitors of the second stage modules areconfigured to be alternatingly connected to two adjacent modules of thefirst stage. In particular, the input capacitor of the fourth modulewould be alternatingly connected to one of the upper two modules of thefirst stage, and the input capacitor of the fifth module would bealternatingly connected to the lower two modules of the first stage.

This principle is not limited to only 5 modules (3 in the first stage+2in the second stage), but can be extended to systems with a much largernumber of modules, for example 19 modules (in which case the supplyvoltage would be divided by 10), or 39 modules (in which case the supplyvoltage would be divided by 20). In general, a system using thistopology would have 2N−1 modules, namely: N modules in the first stage,and (N−1) in the second stage. In this way, systems, e.g. solid statelighting devices can be formed, which are powered from a single DCsupply having a voltage in the range from about 7.5 V to 24V, or from7.5V to about 50V, or from 7.5V to 200V, or from 7.5V to 400V, or from7.5V to 600V, for example for street light applications.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. The mere fact that certain measures are recited in mutuallydifferent dependent claims does not indicate that a combination of thesemeasured cannot be used to advantage. Any reference signs in the claimsshould not be construed as limiting the scope thereof.

1. A system comprising: a first, second and third module each comprising an electronic circuit comprising: an input capacitor connected between a first node and a second node; a first switch and a second switch connected in series between said first node and said second node, in parallel with said input capacitor, and defining an intermediate node at their interconnection; a voltage regulator configured for receiving power from said input capacitor and for providing output power at a configurable voltage between an output node and the second node; wherein the first node of the second module is connected to the second node of the first module; and wherein the intermediate node of the second module is connected to the second node of the third module; and wherein the intermediate node of the first module is connected to the first node of the third module; and wherein the system further comprises a system controller configured for providing a switch control signal of at least 2 MHz to at least the first and the second module, wherein the first and second switch of at least the first and second module are being toggled at a balancing frequency of at least 2 MHz, in such a way that: during a first moment in time, the first switch of the first and second module are configured to be closed while the second switch of the first and second module are configured to be open, and during a second moment in time, the first switch of the first and second module are configured to be open while the second switch of the first and second module A are configured to be closed, thereby causing charge distribution between the input capacitor of the first, and second and third module.
 2. The system according to claim 1, wherein the electronic circuit further comprises an output capacitor connected between the output node and the second node for stabilizing the output voltage.
 3. The system according to claim 1, wherein the electronic circuit further comprises a control unit configured for receiving a switch control signal via a fourth node, and for generating a first switch signal to control the first switch, and for generating a second switch signal to control the second switch.
 4. The system according to claim 3, wherein the control unit is further configured for receiving an output control signal via a fourth node and for providing this output control signal, or a signal derived therefrom to the voltage regulator to control the configurable voltage.
 5. The system according to claim 1, further comprising a light emitting diode connected at the output of the voltage regulator.
 6. The system according to claim 1, wherein the voltage regulator is a linear voltage regulator; or wherein the voltage regulator is a switched inductor converter, or wherein the voltage regulator is a resonant or a hybrid switched capacitor converter.
 7. The system according to claim 6, wherein the voltage regulator is a resonant switched capacitor converter or a hybrid switched capacitor converter comprising at least one capacitor and at least one inductor connected in series with said capacitor, wherein the capacitor has a value in the range from 400 pF to 1.4 nF; wherein the inductor has a value in the range from 40 nH to 160 nH.
 8. The system according to claim 6, wherein the voltage regulator is a resonant switched capacitor converter or a hybrid switched capacitor converter comprising at least two switches, configured to switch at a frequency in the range from 20 MHz to 60 MHz.
 9. The system according to claim 8, wherein each module comprises a light emitting diode; or wherein the system further comprises at least three discrete light emitting diodes, each connected to an output of one module.
 10. The system according to claim 1, wherein the system controller is further configured for providing a first output control signal to the first module, and a second output control signal to the second module, and a third output control signal to the third module.
 11. A display device comprising a plurality of pixels organized in rows and columns, comprising a plurality of systems according to claim 1, each system forming one pixel of said display device. 